a) Field of the Invention
The present invention relates to a process for immobilizing ashes by vitrification thereof in an arc plasma reactor.
In the following description, "ashes" means any kind of ashes or powder material coming from incinerators or from clinkers.
b) Brief description of the prior art
It is well known that the ashes coming from incinerators, in particular those of hospital or waste water treatment plants, may often be classified as "hazardous" because of their high concentration in toxic components, in particular heavy metals.
It is also known that the incineration of domestic sludges or of sludges coming from waste water treatment plants is a method that is more and more used, as the other conventional methods become less and less accessible or acceptable. Such conventional methods include landfill, which is less and less used because of the lack of proper dump sites, spreading of the waste into the fields, and/or disposal of it into the rivers or oceans, which also becomes less and less acceptable because of the pollution that it generates.
Incineration is already used in a very extensive manner in some European countries, because it permits to a substantial reduction in the amount waste, especially sludges. Even for hazardous waste, incineration has become a preferential treatment. However, a problem with this method, is that it generates substantial amount of ashes that must be disposed of. This can only be done by solidification or stabilization of the same, such having to be done in a field that is more and more subject to regulations.
The stabilization treatment that is presently commonly used, consists in stabilizing the waste with a binder consisting of cement, lime, absorbents and thermoplastic or duroplastic materials.
If this method is efficient in some cases, it does not provide full protection. Accordingly, new methods have been developed in order to encapsulate the waste in a more efficient manner.
Amongst these new methods, it has been suggested to immobilize the waste by vitrification thereof, i.e. to proceed to an encapsulation of the waste within glass, in order to isolate the same from the surrounding before disposal of them. This method has first been used for nuclear waste. However, it has also been tried for ashes and is presently being used in an industrial or semi pilot scale by at least two companies, namely DAIDO STEEL and SKF.
In its Japanese plant, DAIDO STEEL proceeds to the vitrification of ashes in an arc plasma reactor that may treat up to 25 tons of waste a day. In SKF premises where the project is still under testing, 250 kg of ashes per hour are vitrified in a blast furnace heated with a plasma torch. In this connection, it is worth mentioning that plasma torches are more and more used for the destruction of waste, because of the high temperature they generate, the high energy density they have, and their low thermal inertia.